1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer, and more particularly to an image forming apparatus which transfers an image formed on an image carrier to a transfer medium borne by a transfer medium bearing member.
2. Related Background Art
Conventionally, a variety of full-color image forming apparatuses have been proposed and put to practical use which comprise a photosensitive drum as an image carrier and a transfer drum as a transfer medium bearing member for bearing the transfer medium such as a paper.
FIG. 10 shows a typical example of such a full-color image forming apparatus. This full-color image forming apparatus has a photosensitive drum 1 supported for free rotation in a direction of the arrow R, around the periphery of which a corona electrifier 2, an optical system 3, a developing device 5, a transfer device 6, and a cleaning unit 7 are disposed along a rotational direction thereof. Also, a transfer cleaning device 9 is disposed obliquely downward of the transfer device 6. The optical system 3, which is comprised of a scanning portion for development and a color separation filter, is an exposure device for exposing a color separated light image E or a light image E corresponding thereto to the photosensitive drum 1, for example, a laser beam exposure device as shown.
The light image E is directed for each separated color onto the photosensitive drum 1 which is uniformly electrified by the electrifier 2 to form an electrostatic latent image. The developing device 5 is a rotational developing device, having four developing units, i.e., a black developing unit 5BK, a cyan developing unit 5C, a magenta developing unit 5M, and a yellow developing unit 5Y, arranged around a central axis 5a, in which the electrostatic latent image on the photosensitive drum 1 is developed by rotating a predetermined developing unit to a developing position opposed to the photosensitive drum 1. This development is done by attaching the toner to the electrostatic latent image on the photosensitive drum 1 with a resin as the base substance to visualize the image (toner image).
Further, a toner image on the photosensitive drum 1 is transferred onto a transfer medium supplied to a position opposed to the photosensitive drum by means of the transfer device 6, the transfer medium being supplied from a transfer medium cassette 101 via a conveyance system along a paper path as indicated by the dotted line in the same figure. The transfer device 6 has a transfer drum 6a, a transfer corona electrifier 6b, an adsorption corona electrifier 6c for the electrostatic adsorption of the transfer medium, an adsorption roller 6d opposed thereto, an inner corona electrifier 6e, and an outer corona electrifier 6f, a transfer medium bearing sheet 6g composed of dielectric being extended cylindrically and integrally therewith over an opening region around the peripheral surface of the transfer drum 6a supported around its axis to be driven for rotation in a direction of the arrow R2. Along with the rotation of the transfer drum 6a, the toner image on the photosensitive drum is transferred successively by the transfer electrifier 6b onto the transfer medium borne on the transfer medium bearing sheet 6g. Onto the transfer medium adsorbed to and conveyed by the transfer medium bearing sheet 6g are transferred a desired number of color images to form a full-color image.
In this manner, if the transfer for the desired number of color images has been terminated, the transfer medium is separated from the transfer drum 6a by separation means 11, and the toner image is fused and fixed by a heat roller fixing unit 12, in which the fixing is performed at so high temperatures that the toner can sufficiently melt to cause the toner of two to four colors to be mixed in the fixing device for the color image forming apparatus. To this end, when the transfer medium passes through the nip portion between a heat roller 12a and a pressure roller 12b, with the front face side onto which the toner is transferred upside, the transfer medium is likely to wrap around the heat roller 12a, resulting in a problem that the toner on the transfer medium is offset to the heat roller 12a. Thus, to prevent the above-mentioned problem, a quantity of silicone oil as the release agent is applied to the heat roller 12a. After the toner image is transferred, the photosensitive drum 1 and the transfer medium bearing sheet 6g lend themselves to the image forming process again after the residual toner left on the surface is cleaned by the cleaning unit 7 and the transfer cleaning device 9 which are cleaning means, respectively.
In the above image forming process, the image is formed on the transfer medium having any of various lengths (extending along the conveyance direction, the same hereinafter). Where the length of the transfer medium is not more than one-half of the peripheral length of the transfer drum 6a, one method has been adopted in which more than one transfer medium is borne on the transfer medium bearing sheet 6g concurrently to form an image, thereby speeding up the throughput in effecting the continuous image formation. For example, where the peripheral length of the transfer drum is long enough to bear two transfer media concurrently, and if the image formation for five sheets is consecutively performed, two sheets--two sheets--one sheet are borne in succession, that is, two sheets are borne at the first and second time, respectively, and one sheet is borne at the third time, to complete the image formation. Since the rotational rate of the transfer drum 6a is substantially constant in making the image formation, the through-put speed is substantially proportional to the rotation number of the transfer drum necessary to form the image on all the transfer media. As above described, when the image formation is performed on five transfer media by bearing two sheets each time, the image formation can be completed in roughly three-fifth the time required for making the image formation by bearing one sheet on the transfer drum 6a each time.
By the way, as shown in FIG. 6, silicone oil Si applied to the heat roller 12a may adhere to the transfer medium P having the toner image fixed thereon in passing through the fixing unit 12. This silicone oil Si is absorbed into the transfer medium P on the side of the surface where no toner image T is formed, but is not absorbed on the opposite face side of the transfer medium P which the toner image T covers like a resin, and resides on the surface of the toner T. Accordingly, when the transfer medium P having the image formed on the first face is caused to form the image on the second face, silicone oil Si adhering to the transfer medium P may be transferred back to the object positioned adjacently on the conveyance passageway, possibly causing a problem as will be described later.
The oil present on the first face of the transfer medium having the image formed on the first face is first transitted onto the transfer medium bearing sheet 6g of the transfer drum 6a. Further, the oil on the transfer medium bearing sheet 6g is transitted onto the surface of the photosensitive drum 1 when the photosensitive drum 1 and the transfer drum 6a are contacted in the transfer unit. By this transition of the oil onto the photosensitive drum 1, the toner deposit may occur if the development is conducted with poor cleaning, significantly reducing the life of the photosensitive drum 1.
At present, various apparatuses having a cleaning device for removing such oil on the transfer medium bearing sheet 6g have been proposed in which the cleaning timing takes place after the transfer of the final color. Accordingly, particularly when the image is formed on both faces of the transfer medium with a plurality of transfer media borne on the transfer drum 6a concurrently, the oil remaining on the transfer medium bearing sheet 6g of the side where no transfer medium is borne may be transitted onto the photosensitive drum 1 upon forming the image on the final one sheet, for example, if the image is formed consecutively on the odd number of sheets, with two sheets borne concurrently. Of course, it is necessary that this oil on the transfer medium bearing sheet 6g be removed by an oil removal cleaner, but this is unpreferable because the color aberration may occur upon a shock given to the transfer drum 6a during the course of the image formation.
Therefore, in particular, if the image formation on both faces is performed consecutively for a plurality of sheets which are borne on the transfer drum 6a concurrently, the oil remains on the surface of the photosensitive drum 1, and thereafter, if the image formation is normally performed one sheet each time, the toner may be attached to the area originally white, owing to the adhesion of the oil, appearing as the fog, while in the area which is originally the solid part, the toner may not be fully transferred from the photosensitive drum i onto the transfer medium, also owing to the adhesion of the oil, resulting in a problem of producing a vague image.
To prevent this, it was previously practiced to cut off the transfer current on the course of copying one sheet, or release the pressure of the transfer 1 medium bearing sheet against the image carrier, but which exhibited less sufficient effects and often caused image degradation due to the shock.